U.S. Pat. No. 7,080,595 B2 discloses a manually operated press into which a backstroke inhibitor is electronically implemented, such as known e.g. from U.S. Pat. No. 6,755,124 B2.
Manually operated presses of the kind described above are conventionally used for piece-work workplaces. As in manually operated presses the exerted force increases towards the end of the pressing stroke, some operating persons tend to exert too much force and, thereby, produce pressed workpieces of bad quality or even damaged workpieces.
For many conventional manually operated presses no documentation is produced, in contrast to automatically executed pressing operations for which numerically controlled presses are conventionally used. The lack of documentation, however, is nowadays no longer acceptable for many fields of application, in particular when production processes are to be certified under the ISO 9000 standard.
In order to make sure that one can distinguish between “good” and “bad” parts, European patent specification 0 960 017 B1 teaches to provide a press with a sensor for the pressing stroke (displacement position) as well as with another sensor for the pressing force for generating a displacement-force diagram for any pressing operation being characteristic for a good and for a bad pressing. If a measured displacement-force curve lies within a given tolerance band, then the respective part is identified as well pressed. If not, the part is identified as a bad part that must be disposed of.
In order to be able to determine the pressing force exerted during a pressing operation, e.g. on a Seeger circlip ring, a bearing, a pinion, a sealing etc., a press ram of the press is configured as a force sensor. The force sensor has a force measuring system, for example a strain gauge strip, integrated therein. The strain gauge is connected to a press control unit of the press which, in turn, may be connected with a rotation sensor, for example, for sensing the rotation angle of the actuation lever and, hence, the ram displacement position. The control unit then processes the sensed data to enable the above-mentioned differentiation between good and bad parts once the pressing operation is executed.
If the examination shows that a bad part was pressed, the press may switch off automatically when the bad part is still within the press. This is likewise made upon a respective command from the control unit.
To start with, there is a first problem that during conventional manual pressing operations, in contrast to automatically executed pressing operations, there may unintentionally occur high pressing forces which, for example, are caused by negligence of the operating person. High pressing forces may likewise occur when the parts which are to be pressed together, are already sufficiently joined, however, the mechanical final position of the pressing stroke has not yet been reached. In such a situation, the operating person “feels” that the actuation lever may still be moved further in the pressing direction, and, therefore executes such movement to its end. In such a case a too high pressing force may be executed resulting in a “bad” pressing.
It is, therefore, necessary that the exerted pressing force be measured as precisely as possible in order to be able to optimally execute the quality examination based thereon. For that purpose highly sensitive force measuring systems are used which, however, are destroyed or at least damaged at too high mechanical overloads. Moreover, high overloads may occur from time to time that exceed admissible overload specifications by 100 or 200%.
As already mentioned above, the prior art teaches to record the force as a function of the displacement position for making a quality evaluation (good/bad) once the pressing operation is finished. As the pressing operation is effected manually, each pressing operation is effected with different pressing force. For that reason some work pieces which shall be pressed together, may already be joined sufficiently “well” before the press ram has reached its mechanical final position, or some work pieces may be sufficiently joined only at the final position. In the event that the sufficient joining has already been achieved prematurely, it would be desirable to abort the pressing operation before the final (mechanical) press ram position has been reached.